The present invention relates to differential circuits and techniques, and more particularly to a bias circuit for low voltage differential applications.
The bipolar junction transistor (BJT) differential pair is one of the most important integrated circuit (IC) building blocks. The emitters of two NPN BJ transistors Q1, Q2 are connected together and to a constant current sink to ground. The supply voltage is connected to the collectors of the transistors through separate bias resistors, which may have matching resistances. A differential input signal (VIN+, VINxe2x88x92) is applied across the bases of the differential pair, and a differential output voltage (VOUT+, VOUTxe2x88x92) is developed across the collectors of the differential pair. The current sink establishes a bias current IBIAS that sets the total of the sum of emitter currents for transistor differential pair Q1 and Q2. Usually, the base currents are relatively small (i.e., high current gain) so the sum of the collector currents (IC1 and IC2) of the transistors Q1 and Q2 is also equal to the current from the current sink. This results in the well-known hyperbolic tangent function xe2x80x9ctanhxe2x80x9d for the differential collector currents as illustrated by the following equations 1 and 2:
(IC1+IC2)=IBIASxe2x80x83xe2x80x83(EQ 1)
(IC1xe2x88x92IC2)=(IBIAS) tanh [VD/2VT], VD=(VIN+xe2x88x92VINxe2x88x92)xe2x80x83xe2x80x83(EQ 2)
where VT is a thermal coefficient voltage (the voltage equivalent of temperature), where VT=kT/q, where xe2x80x9ckxe2x80x9d is the Boltzmann constant in joules per degree Kelvin, T is the temperature in degrees Kelvin (absolute scale), and xe2x80x9cqxe2x80x9d is the magnitude of the charge of an electron.
The current sink bias sets the value of the sum of the collector currents of the transistors. The differential collector current depends upon the differential base voltages, but its maximum value is also limited by the current sink bias. A DC supply of base current is usually required for this circuit to work. Sometimes the DC base current is supplied by the signal source itself, but if the signal is AC coupled to the base, as is often the case for radio frequency (RF) circuits, for example, then a separate DC bias path is used to supply DC base current. In this case, the DC base bias circuit usually has an AC impedance value that is large compared to the impedance of the base terminals of the BJTs, so that little signal is lost to this base bias circuit.
For circuits operating with supply voltages less than 3 volts (V), the differential pair uses a large percentage of the total available supply voltage. The current sink itself may consume nearly a volt.
A bias circuit according to embodiments of the present invention provides biasing for a differential circuit, where the differential circuit includes a pair of differential transistors having control inputs receiving a differential signal and current paths coupled between a source signal and ground. The bias circuit includes first and second transistors, first and second impedance devices, a reference current source and an amplifier. The first and second transistors each have a control input and further have a current path coupled between a first node and ground. The control inputs of the first and second transistors receive the differential signal. The first and second impedance devices are each coupled between a control input of one of the first and second transistors and a second node. The reference current source provides a reference current for the first node and the amplifier has an input coupled to the first node and an output coupled to the second node. In this manner, the differential circuit need not include a constant current source in series with the differential transistors so that the differential circuit may be utilized in low voltage applications.
An inductor may be coupled in the current path of the differential transistors between the first and second transistors and ground. The impedance of an inductor is low at low frequencies and thus has little or no effect. The impedance of the inductor is, however, high at high frequencies. In this manner, the inductor operates in a similar manner as a constant current source at high frequencies so that the circuit responds in a similar manner as a standard differential circuit. A filter capacitor may be coupled between the first node and ground and operates as a low pass filter.
In more specific configurations, the first and second transistors each comprise a bipolar junction transistor (BJT) having a base input and further having a current path formed by a collector and an emitter. Also, the first and second transistors may each comprise NPN transistors. Also, the emitters of the differential transistors of the differential circuit and the first and second transistors may each be connected to ground. The differential transistors of the differential circuit may also be BJTs, where the first and second transistors have emitter area ratios that are matched to the emitter area ratios of the differential transistors of the differential circuit. It is noted that PNP or metal oxide semiconductor transistor (MOS) configurations are also contemplated.
The first and second impedance devices may be resistors or inductors. The reference current may be fixed, or may be modulated to make the circuit into a type of mixer or multiplier. Also, the impedance devices may be replaced by current sources or mixers, which are controlled by either of the first or second nodes. Further, additional pairs of transistors may be added to obtain additional pairs of output currents. Emitter generation may be added between each transistor emitter and ground. The amplifier may be a non-inverting amplifier, a buffer, an emitter-follower, or even a short circuit in some configurations.
A biased differential circuit according to the present invention includes first and second pairs of differential transistors, first and second resistors, first and second impedance devices, a reference current source and an amplifier. The first pair of differential transistors each has a control input and first and second current terminals, where the first terminal of each is coupled to ground and where the control inputs of the pair of differential transistors receive the differential input signal. The first and second resistors are each coupled between the source signal and the second current terminal of a respective one of the first pair of differential transistors. The second current terminals of the first pair of differential transistors develops a differential output signal. The second pair of differential transistors each have a control input and first and second current terminals coupled between a first node and ground. The control inputs of the second pair of differential transistors receive the differential input signal. The first and second impedance devices are each coupled between a control input of one of the second pair of differential transistors and a second node. The reference current source is coupled to the source signal and provides a reference current for the first node. The amplifier has an input coupled to the first node and an output coupled to the second node. The variations previously described are applicable in a similar manner.